There are two types of nuclear weapons, each of which utilizes a different nuclear reaction: nuclear fission and nuclear fusion. The bomb developed by the Manhattan Project and dropped on Japan during World War II were fission weapons, also known as atomic bombs. Hydrogen bombs are fusion weapons, and these were first developed and produced during the early 1950s.

In fission weapons, the explosive energy is derived from nuclear fission, in which large atomic nuclei are split into two roughly equal parts. Every time a nucleus divides, it releases a large amount of energy. Each fission reaction also produces one or more neutrons, the subatomic particles that are needed to initiate a fission reaction. Thus, once a fission reaction has been started in a few nuclei, it rapidly spreads to other nuclei around it, creating a chain reaction. The two nuclei most commonly used for this...